After the shooting, investigators searched a safe connected to the shooting suspect, Jared Lee Loughner, and found a letter apparently sent to him by Ms. Giffords's office thanking him for previously attending a similar "Congress on your corner" event in 2007.
Much remains unknown about what motivated Mr. Loughner, who is in custody. But the initial evidence, including the constituent letter, has led law enforcement officials to think that the suspect had been thinking about the congresswoman for years, according to people familiar with the case.